This patch changes the mechanism by which the client enables a 
plugin. Instead of using INSERT IGNORE to reload a plugin library,
it now uses REPLACE INTO. This allows users to load a library
multiple times replacing the existing values in the mysql.plugin
table. This allows users to replace the symbol reference to a
different dl name in the table. Thus permitting enabling of 
multiple versions of the same library without first disabling
the old version.

A regression test was added to ensure this feature works.